In the technical realm, many applications in a wide array of sectors call for high-strength sheet metal parts that are lightweight. For instance, the automotive industry is striving to reduce the fuel consumption of motor vehicles so as to lower the CO2 emissions while, at the same time, improving passenger safety. For this reason, there is an ever-growing demand for autobody parts that have a favorable strength-to-weight ratio. These parts especially include A and B pillars, side-impact bars in doors, rocker panels, frame parts, bumpers, crossbeams for the floor and roof as well as front and rear longitudinal beams. In modern motor vehicles, the bodyshell with a safety cage is usually made of hardened sheet steel with a strength of about 1500 MPa. Al—Si-coated steel sheets are often used for this. The process of so-called press hardening has been developed for purposes of manufacturing parts made of hardened sheet steel. Here, steel sheets are first heated up to the austenitic temperature between 850° C. and 950° C. [1562° F. and 1742° F.], then placed into a pressing die, quickly formed and rapidly quenched by the water-cooled die to the martensitic temperature of approximately 250° C. [482° F.]. This gives rise to a hard, strong martensitic structure with a strength of about 1500 MPa. A steel sheet hardened in this manner, however, only has an elongation at break of about 6% to 8%, which is a drawback in certain areas if two vehicles collide, especially in the case of a side impact. The kinetic energy of the impacting vehicle cannot be converted into deformation heat. Rather, in this case, the part will undergo brittle fracture, additionally posing a risk of injury to the passengers.
For this reason, the automotive industry is striving to develop autobody parts that have several, different elongation and strength zones so that one single part can have very strong areas on the one hand and very extensible areas on the other hand. In this context, the general requirements made of a production installation should also be taken into account: for instance, the cycle time of the press hardening installation should not be detrimentally affected, it should be possible to use the entire installation universally without restrictions and to quickly retool it according to customer specifications. The process should be robust and cost-efficient, and the production installation should only take up a minimal amount of space. The shape and the edge precision of the part should be so high that the need for hard-trimming the hardened part is virtually eliminated, thus saving material and work.
The state of the art describes such methods and devices. In this context, these methods make use of partially heated dies, whereby one area of the part is cooled off above the martensite-forming quenching velocity. The rest of the part is cooled off abruptly as is normally done, thereby forming martensite. European publication EP 2 012 948, for example, describes a forming die for press-hardening and for the temperature-controlled forming of a blank consisting of high-strength and/or ultra-high strength steel grades; this die that has means for controlling the temperature of the forming die and this publication also describes a method for press-hardening and for temperature-controlled forming of a blank consisting of high-strength and/or ultra-high strength steel grades in which the blank is heated prior to the forming process and subsequently formed in a forming die while it is hot or warm, whereby the forming die has means for controlling the temperature. Here, several temperature-control means are provided in the forming die, as a result of which a plurality of temperature zones can be defined, whereby at least the contact surfaces of the die elements used for the forming process are associated with individual temperature zones.
German patent document DE 10 2005 032 113 discloses a device and a method for hot-working and partially hardening a part positioned between two die halves in a press. The die halves are each divided into at least two segments that are separated from each other by thermal insulation. The two segments can be heated or cooled by means of a temperature-control unit, so that different temperatures and thus different cooling curves can be established in different areas of the part. This makes it possible to manufacture a part with areas of different hardness and ductility.
International patent document WO 2009/113 938 describes a press-hardening process with which soft areas can be created in the finished product by reducing the cooling rate of these material sections. This diminishes the martensite fraction in these areas and consequently increases the elongation at break of these areas.
In this context, all of the methods that use a partially heated die entail the drawback that the part becomes warped since the part is removed from the die with partially different temperatures ranging from about 300° C. to 500° C. [572° F. to 932° F.] in the soft area, and of about 100° C. [212° F.] in the martensitic areas, after which it is further cooled away from the fixed shape of the die. Moreover, the cycle time of the process is lengthened since the fast cooling is slowed down in order to promote pearlite-ferrite formation, as a result of which the cost-effectiveness is likewise reduced. In addition, such dies are very complex and therefore expensive and malfunction-prone.
In another method known from the state of the art, for example, German patent documents DE 10 350 885, DE 10 240 675, DE 10 2005 051 403 or DE 10 2007 012 180, in a dual-zone furnace, the soft area of the part is heated up to a temperature below the material-dependent Ac3 temperature, whereas the area that is to be hardened, in contrast, is heated up to a temperature above the Ac3 temperature. In this process, an extensible soft pearlite-ferrite is formed in one area of the part and a hard martensite is formed in another area of the part. The disadvantage of this process is that the furnace can only be employed with certain limitations and can no longer serve as a universal furnace. This translates into a loss of cost-effectiveness for this method. Another disadvantage is that the separation of the areas usually cannot be accomplished with sufficient precision over the long run. Moreover, it is not feasible to implement more than two different zones. Furthermore, when Al—Si-coated parts are used, the temperature has to be kept at approximately 950° C. [1742° F.] for about 300 seconds so that the coating can diffuse into the base material. This process takes considerably longer at lower temperatures, thus reducing the cost-effectiveness of the entire installation.
Moreover, another method is known in actual practice in which the soft areas are partially cooled slowly. In this process, the entire part is heated up above the austenitic temperature beyond the diffusion time and diffusion temperature, and subsequently, either in a separate furnace or in the same furnace, it is cooled down again slowly to below the austenitic temperature in that it is partially exposed to air. When the press-hardening process is subsequently carried out in the die, the drawbacks in terms of the insufficient dimensional precision and the cost-effectiveness of the production furnace are eliminated. A disadvantage of this method is the slower cycle time caused by the additional work step. Yet another disadvantage is the undefined cooling rate that occasionally leads to martensite formation in parts that are less than 1.2 mm-thick. The cooling rate is undefined because the cooling takes place at an ambient temperature that cannot be precisely defined. For this reason, the process cannot be said to be robust. Moreover, this process can only be carried out with two zones of different hardness.
European Preliminary Published Application EP 2 143 808 A1 describes a method for the production of a shaped part having at least two structural areas of different ductility from a blank made of hardenable steel, different areas of which are heated differently and subsequently shaped in a heat-forming and hardening die and then hardened in certain areas, and also having an infrared lamp array. The blank made of hardenable steel is heated in a heating device to a homogeneous temperature that is lower than the Ac3 point of the alloy. Subsequently, the infrared lamp array is used to bring areas of the blank that are of the first type to a temperature above the Ac3 point of the alloy, and hardened in a heat-forming and hardening die in the areas of the first type. The result is a shaped part made of steel and having at least two structural areas of different ductility. The appertaining furnace system has a profiling furnace with one level, whereby the one level has an upper section and a lower section as well as a receptacle for a product-specific intermediate flange and the product-specific intermediate flange installed in it. Here, the product-specific intermediate flange is designed to impart a prescribed temperature profile to the part with temperatures above the Ac3 temperature for an area that is to be hardened and below Ac3 for a more ductile area.
Finally, German patent specification DE 10 2009 051822 B3 discloses a method for the production of shaped sheet metal parts made of high-strength steel and having partially differing strength properties in which a blank is heated up to a temperature that is higher than an Ac3 temperature, whereby the blank heated in this manner is subsequently fed into a forming die, where it is shaped and quenched, whereby it is preferably provided that partial zones of the shaped sheet part are merely annealed by controlling the temperature. In order to create a simpler and more efficient method, after being heated, the blank is partially cooled to a defined temperature, especially to a temperature below the Ac3 temperature, in an upstream conveying installation having upper and/or lower coolable conveying rollers.
Finally, it is also possible to weld different grades of steel together, so that unhardenable steel is present in the soft zones while hardenable steel is found in the hard zones. During the subsequent hardening process, the desired hardness profile is achieved over the entire part. The drawbacks of this process are the occasionally unreliable weld seam of the approximately 0.8 to 1.5 mm-thick Al—Si-coated sheet metal normally used for chassis parts, the abrupt hardness transition there as well as the increased costs of the sheet metal due to the additional production step of welding. During testing, failures occasionally occurred due to breakage in the vicinity of the weld seam, so that the process cannot be considered to be robust.